Strictly speaking, this means cooking food in boiling water (A liquid is boiling when the surface is continually agitated by large bubbles). In practice, however, many 'boiled' foods are cooked at a temperature below boiling point, that is, they are simmered or poached. All meat, fish, poultry and stews should be simmered. Vegetables, peas, puddings, beans, lentils, etc are boiled. Sufficient heat should be applied to keep the liquid bubbling gently. Violent boiling should be avoided. It wastes fuel; it does not cook the food any faster, it tends to make the food break up and so spoils the appearance; the liquid is evaporated too quickly with the consequent danger of the food burning. There are one or two exceptions to this rule; for example, when one wants to drive off water quickly from syrup or a sauce to make it thicker, then violent boiling with the lid off hastens the process.

Boiling, a type of phase transition, is the rapid vaporization of a liquid, which typically occurs when a liquid is heated to its boiling point, the temperature at which the vapor pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding atmospheric pressure. Thus, a liquid may also boil when the pressure of the surrounding atmosphere is sufficiently reduced, such as the use of a vacuum pump or at high altitudes. Boiling occurs in three characteristic stages, which are nucleate, transition and film boiling.

Wort is boiled in the brew kettle, typically for 90 minutes. During this time, the wort is spiced with hops. Boiling stops all mash enzyme activity, it extracts bitter and aromatic substances from the hops, it boils off any harsh grainy odors, and it precipitates the trub which helps clarify the wort.

The process of creating a stable wort for fermentation. Both finishing and flavoring hops are added during various stages of the boil to add their flavors. It also causes tannins and proteins to coagulate out.